What I would do: Seal the grout (if you haven't already) with a sealer that says it's for outdoor use. Seal the inside of the clay pot. Then find a plastic pot that fits inside your mosaic pot and place your plant inside that liner pot. Then leave outside and see what happens! What has happened to my mosaic pots is that over time, the plant water leaches out through the grout and discolors it.

Someone on garden junk (Jan in Ca I think) seals her pots with spray car undercoating. Maybe if you let the pot dry out really well you could try that on the inside. Has to be very dry or the moisture could be trapped inside espceilally if you have already sealed the outside. Good luck.
Kathy